Selectable current limiting for power tool

ABSTRACT

Selectable current limiting for a power tool. One embodiment provides a method for selectable current limiting for a power tool including determining, using a current sensor, an average current and determining whether the average current exceeds a predetermined current threshold. The method also includes determining a deviation of the average current from the predetermined current threshold and reducing a PWM duty ratio proportional to the deviation of the average current from the predetermined current threshold. The PWM duty ratio corresponds to a PWM signal provided to an inverter bridge.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/946,072, filed on Dec. 10, 2019, the entire contentof which is hereby incorporated by reference.

FIELD

The present disclosure relates to selectable power limiting in powertools, for example, in powered handheld pruners.

BACKGROUND

Hand-held power tools include a motor for operation of a tool bit. Thecurrent drawn by the motor varies based on the load on the tool bit. Forexample, more current may be drawn in high load situations (e.g.,cutting through hard, thick wood) than in low load situations (e.g.,cutting through soft, thin wood).

SUMMARY

Some embodiments provide a power tool including a housing, a motorwithin the housing, a current sensor configured to measure a currentflow to the motor, an inverter bridge selectively providing power from abattery pack to the motor, and an electronic processor coupled to thecurrent sensor and the inverter bridge. The electronic processor isconfigured to determine, using the current sensor, an average currentand determine whether the average current exceeds a predeterminedcurrent threshold. The electronic processor is also configured todetermine a deviation of the average current from the predeterminedcurrent threshold and reduce a PWM duty ratio proportional to thedeviation of the average current from the predetermined currentthreshold. The PWM duty ratio corresponds to a PWM signal provided tothe inverter bridge.

Some embodiments provide a method for selectable current limiting for apower tool including determining, using the current sensor, an averagecurrent and determining whether the average current exceeds apredetermined current threshold. The method also includes determining adeviation of the average current from the predetermined currentthreshold and reducing a PWM duty ratio proportional to the deviation ofthe average current from the predetermined current threshold. The PWMduty ratio corresponds to a PWM signal provided to the inverter bridge.

Some embodiments provide a power tool including a housing, a motorwithin the housing, a current sensor configured to measure a currentflow to the motor, an inverter bridge selectively providing power from abattery pack to the motor, and an electronic processor coupled to thecurrent sensor and the inverter bridge. The electronic processor isconfigured to determine, using the current sensor, an average currentand determine whether the average current exceeds a predeterminedcurrent threshold. The electronic processor is also configured todetermine a deviation of the average current from the predeterminedcurrent threshold and reduce a PWM duty ratio based on the deviation ofthe average current from the predetermined current threshold. The PWMduty ratio corresponds to a PWM signal provided to the inverter bridge.

Before any embodiments are explained in detail, it is to be understoodthat the embodiments are not limited in its application to the detailsof the configuration and arrangement of components set forth in thefollowing description or illustrated in the accompanying drawings. Theembodiments are capable of being practiced or of being carried out invarious ways. Also, it is to be understood that the phraseology andterminology used herein are for the purpose of description and shouldnot be regarded as limiting. The use of “including,” “comprising,” or“having” and variations thereof are meant to encompass the items listedthereafter and equivalents thereof as well as additional items. Unlessspecified or limited otherwise, the terms “mounted,” “connected,”“supported,” and “coupled” and variations thereof are used broadly andencompass both direct and indirect mountings, connections, supports, andcouplings.

In addition, it should be understood that embodiments may includehardware, software, and electronic components or modules that, forpurposes of discussion, may be illustrated and described as if themajority of the components were implemented solely in hardware. However,one of ordinary skill in the art, and based on a reading of thisdetailed description, would recognize that, in at least one embodiment,the electronic-based aspects may be implemented in software (e.g.,stored on non-transitory computer-readable medium) executable by one ormore processing units, such as a microprocessor and/or applicationspecific integrated circuits (“ASICs”). As such, it should be noted thata plurality of hardware and software based devices, as well as aplurality of different structural components, may be utilized toimplement the embodiments. For example, “servers,” “computing devices,”“controllers,” “processors,” etc., described in the specification caninclude one or more processing units, one or more computer-readablemedium modules, one or more input/output interfaces, and variousconnections (e.g., a system bus) connecting the components.

Relative terminology, such as, for example, “about,” “approximately,”“substantially,” etc., used in connection with a quantity or conditionwould be understood by those of ordinary skill to be inclusive of thestated value and has the meaning dictated by the context (e.g., the termincludes at least the degree of error associated with the measurementaccuracy, tolerances [e.g., manufacturing, assembly, use, etc.]associated with the particular value, etc.). Such terminology shouldalso be considered as disclosing the range defined by the absolutevalues of the two endpoints. For example, the expression “from about 2to about 4” also discloses the range “from 2 to 4”. The relativeterminology may refer to plus or minus a percentage (e.g., 1%, 5%, 10%,or more) of an indicated value.

It should be understood that although certain drawings illustratehardware and software located within particular devices, thesedepictions are for illustrative purposes only. Functionality describedherein as being performed by one component may be performed by multiplecomponents in a distributed manner. Likewise, functionality performed bymultiple components may be consolidated and performed by a singlecomponent. In some embodiments, the illustrated components may becombined or divided into separate software, firmware and/or hardware.For example, instead of being located within and performed by a singleelectronic processor, logic and processing may be distributed amongmultiple electronic processors. Regardless of how they are combined ordivided, hardware and software components may be located on the samecomputing device or may be distributed among different computing devicesconnected by one or more networks or other suitable communication links.Similarly, a component described as performing particular functionalitymay also perform additional functionality not described herein. Forexample, a device or structure that is “configured” in a certain way isconfigured in at least that way but may also be configured in ways thatare not explicitly listed.

Other aspects and aspects of the disclosure will become apparent byconsideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a power tool in accordance with someembodiments.

FIG. 2 is a block diagram of the power tool of FIG. 1 in accordance withsome embodiments.

FIG. 3 is a flowchart of a method for selectable current limiting forthe power tool of FIG. 1 in accordance with some embodiments.

FIG. 4 is a graphical illustration of the relationship between averagecurrent, current threshold, PWM settings of the power tool of FIG. 1when current limiting is enabled in accordance with some embodiments.

DETAILED DESCRIPTION

As noted above, current drawn by a power tool motor may vary based onthe load on a driven implement of the tool, such as a driver bit, adrill bit, a saw chain (e.g., on a chain saw), a saw blade, and thelike. For example, more current may be drawn in high load situations(e.g., cutting through hard, thick wood) than in low load situations(e.g., cutting through soft, thin wood). In battery powered power tools,excess current draw may result in reduced run time of the power tool ordamage to the battery pack.

FIG. 1 illustrates one example embodiment of a power tool 100. In theexample illustrated, the power tool 100 is a powered handheld pruner ora portable chainsaw. The chainsaw 100 is powered by a rechargeable powertool battery pack 110. For example, the illustrated battery pack 110 isan interchangeable battery pack configured to connect to and power avariety of tools in addition to the chainsaw 100. In the illustratedembodiment, the battery pack 110 includes a stem 120. In otherembodiments, the battery pack 110 is a slide-type battery back and doesnot include the stem 120. In some embodiments, the battery pack 110 is a12-volt lithium-ion battery pack including three battery cells (notshown) connected in series. In other embodiments, the battery pack 110may include fewer or more battery cells, and the battery pack 110 mayhave other nominal output voltages, such as 14.4 volts, 18 volts, etc.Additionally or alternatively, the battery cells may have chemistriesother than lithium-ion such as, for example, nickel cadmium, nickelmetal-hydride, or the like. In still other embodiments, the chainsaw 100may be a corded power tool.

With continued reference to FIG. 1, the chainsaw 100 includes a housing130. The housing 130 defines a handle housing portion 140, a motorhousing portion 150 and a drive housing portion 160. In the illustratedembodiment, the handle housing portion 140 extends from the drivehousing portion 160 and the motor housing portion 150. In otherembodiments, the handle housing portion 140 may extend from either thedrive housing portion 160 or the motor housing portion 150. In theillustrated embodiment, the handle housing portion 140 includes abattery receiving portion 170 disposed opposite the motor housingportion 150. At least a portion of the battery pack 110 may be coupledto the battery receiving portion 170. In other embodiments, the batteryreceiving portion 170 may be defined elsewhere on or within the housing130.

Referring to FIG. 1, the illustrated housing 130 further includes ahandle guard 180 that extends between the drive housing portion 160 andthe battery receiving portion 170. The handle housing portion 140includes at least one grip surface 190 for a user to grasp whileoperating the chainsaw 100. The handle guard 180 may support removableadjusting tools or buttons for adjusting settings on the chainsaw 100. Aswitch 200 is positioned on the handle housing portion 140 for operatingthe chainsaw 100. As illustrated, the switch 200 is an on/off triggerswitch. In other embodiments, the switch 200 may be a variable speedtrigger switch, a two speed trigger switch, a push button, or anothersuitable actuator. An additional current limiting switch 210 is providedon the handle housing portion 140 close to the switch 200 and accessibleby the user such that the switch 200 and the current limiting switch 210may be operated using one hand of the user. The current limiting switch210 is, for example, a two position switch for enabling or disabling thecurrent limiting feature as described below. In some embodiments, thecurrent limiting switch 210 may be provided at other locations, forexample, on the handle guard 180, on the drive housing portion 160, onmotor housing portion 150. In other embodiments, the current limitingswitch 210 may be provided outside the tool, for example, on a graphicaluser interface of a smart telephone executing a software application,where the smart telephone is in wireless communication with the chainsaw100 via respective compatible wireless communication interfaces (e.g.,Bluetooth® or WiFi® transceivers) on the smart telephone and chainsaw100.

With reference to FIG. 2, the chainsaw 100 includes an electronicprocessor 220, a memory 230, a motor 240, an inverter bridge 250, adischarge switch 260, and a current sensor 270. The electronic processor220 may be implemented as, for example, a microprocessor, amicrocontroller, a field programmable gate array, an applicationspecific integrated circuit, or the like. The memory 230 may be part ofthe electronic processor 220 or may be a separate component. The memory230 may include, for example, a program storage area and a data storagearea. The memory 230 stores executable instructions that when executedby the electronic processor 220, cause the chainsaw 100 to perform thefunctions described herein. For example, the electronic processor 220controls the motor 240 and the current supply between the battery pack110 and the motor 240. The electronic processor 220 and the memory 230together form an electronic controller.

The motor 240 may be a brushless direct current motor. The inverterbridge 250 includes a plurality of field effect transistors (FETs)coupled between the battery pack 110 and the motor 240. The electronicprocessor 220 controls, for example, through a gate driver (not shown)that may be separate or incorporated into the electronic processor 220,a pulse width modulated cycle of the plurality of FETs to operate andcontrol the speed of the motor 240. The electronic processor 220 may useclosed-loop speed control, open-loop speed control, or a combination ofthe two to operate the motor 240. Particularly, in the presentapplication, operating the motor 240 at a selected speed may includeoperating the motor 240 at a particular speed using closed-loop speedcontrol, operating the motor 240 at a particular duty cycle usingopen-loop speed control, or the combination of the two. In closed-loopspeed control, for example, the electronic processor 220 receives adesired speed (for example, via variable speed trigger switch 200),drives the inverter bridge 250 at an initial pulse width modulated (PWM)duty cycle, detects speed of the motor 240 (for example, using Hallsensors configured to detect rotation of the rotor magnets), and adjuststhe PWM duty cycle up or down to achieve the desired speed. In open loopspeed control, for example, the electronic processor 220 receives adesired speed (for example, proportional to a depression amount ofvariable speed trigger switch 200), accesses a lookup table stored inthe memory 230 to obtain a PWM duty cycle mapped to the desired speed,and drives the inverter bridge 250 at the PWM duty cycle obtained fromthe memory. In some embodiments, where the switch 200 is an ON/OFFswitch, the electronic processor 220 sets the desired speed based on theload on the motor 240 or other criteria as described below and stored inthe memory 230.

The power tool 100 includes a transmission (in the drive housing portion160) to transfer the rotation motion of the motor 240 to the driven toolimplement (for example, the chainsaw or pruner chain). Accordingly, bycontrolling the motor speed (e.g., via open- or closed-loop speedcontrol), the tool output is also controlled. The power tool 100 isactivated by actuating the switch 200. Actuating the switch 200 to theON position may close the discharge switch 260 allowing current to flowfrom the battery pack 110 to the motor 240. Similarly, actuating theswitch 200 to the OFF position may open the discharge switch 260terminating the current flow between the battery pack 110 and the motor240. In some embodiments, rather than directly controlling the dischargeswitch 260, the switch 200 provides a signal to the electronic processor220, which in turn controls the discharge switch 260 based on the signalreceived from the switch 200.

The current sensor 270 measures current flowing to the motor 240 andprovides an indication of the amount of current flow to the electronicprocessor 220. In some embodiments, the current sensor 270 continuouslyprovide an indication of instantaneous current flowing to the motor 240.In other embodiments, the current sensor 270 provides the indication ofinstantaneous current at discrete time intervals. The electronicprocessor 220 receives the indications and may determine an averagecurrent over a predetermined time period. In some embodiments, a slowresponse filter (e.g., a low-pass filter), a fast response filter, acombination of the slow and fast response filter may be provided betweenthe current sensor 270 and the electronic processor 220. The filter,continuously or at discrete time intervals, provides an indication ofthe average current to the electronic processor 220.

Operating the chainsaw 100 at high current draw may quickly deplete thebattery resulting in reduced runtime of the chainsaw 100. Operating thechainsaw 100 at high current draw for long periods of time may alsogenerate heat in the chainsaw 100. In some embodiments, the electronicprocessor 220 may shut off the tool when the current draw exceeds apredetermined threshold to extend the run time and reduce heatgeneration. However, this deteriorates the user experience of the powertool 100. Particularly, frequent stopping and starting of the chainsaw100 may cause annoyance to the user.

FIG. 3 is a flowchart of an example method 300 for selective currentlimiting of the chainsaw 100. While described with respect to thechainsaw, the method is also applicable to other power tools (e.g.,drill-drivers, impact tools, reciprocating saws, circular saws, mitersaws, sanders, and the like). The method 300 includes determining, usingthe current sensor 270, an average current (at block 310). Theelectronic processor 220 determines the average current based on theindications received directly from the current sensor 270 or through anaverage filter. The electronic processor 220 may determine the averagecurrent from startup of the tool to the instant of current measurement.Alternatively, the electronic processor 220 may determine a movingaverage of current at every measurement instance or average current overa predetermined time period. The moving average is, for example, aslow-moving current average determined by passing the current sensor 270readings through a slow response filter or a fast-moving current averagedetermine by passing the current sensor 270 readings through a fastresponse filter. The slow response filter determines an average currentover a longer period of time than the fast response filter. In someembodiments, the electronic processor 220 determines different averagesbased on the fast-moving current average and the slow-moving currentaverage.

The method also includes determining, using the electronic processor220, whether the average current exceeds a predetermined currentthreshold (at block 320). The electronic processor 220 compares theaverage current to the predetermined current threshold stored in thememory 230. When the average current exceeds the predetermined currentthreshold, the electronic processor 220 proceeds to block 310 todetermine the next instance or interval of average current. Theelectronic processor 220 then includes determining a deviation of theaverage current from the predetermined current threshold (at block 330)when the average current exceeds the predetermined current threshold. Insome embodiments, the electronic processor 220 determines the averagecurrent using software executing a comparison. In some embodiments, theelectronic processor 220 determines the average current using anexternal comparator that receives and compares the predetermined currentthreshold and the average current. The comparator then sends to theelectronic processor 220 an indication of how much the average currentdeviates from the predetermined current threshold.

The method 300 includes reducing, using the electronic processor 220, aPWM duty ratio proportionally to the deviation of the average currentfrom the predetermined current threshold (at block 340). The electronicprocessor 220 reduces the PWM duty ratio of the PWM signals provided tothe plurality of FETs in the inverter bridge 250. The amount by whichthe PWM duty ratio is reduced is based on (e.g., proportional to) theamount by which the average current deviates from the predeterminedcurrent threshold. In some embodiments, reducing the PWM duty ratioproportional to the deviation includes reducing the PWM duty ratio insteps. For example, for 0-10% of maximum deviation, the PWM duty ratiomay be reduced by 10%, for 10-20% of maximum deviation, the PWM dutyratio may be reduced by 20%, and so on. In other embodiments, reducingthe PWM duty ratio proportional to the deviation includes reducing thePWM duty ratio continuous with the percentage of maximum deviation.Maximum deviation is, for example, the difference between an absolutecurrent limit of the chainsaw 100 and the predetermined currentthreshold.

In one example, the chainsaw 100 includes a PWM duty ratio maximumlimit. The PWM duty ratio maximum limit is for example 100%. When thechainsaw 100 is turned on or after a soft start period after beingturned on, the chainsaw 100 operates at a speed corresponding to the PWMduty ratio maximum limit. The chainsaw 100 also includes a PWM dutyratio lower limit. The PWM duty ratio lower limit is, for example, 30%.In some embodiments, the electronic processor 220 does not reduce thePWM duty ratio below the PWM duty ratio lower limit, even when thedeviation between the average current and the predetermined currentthreshold exceeds the difference between the PWM duty ratio maximumlimit and the PWM duty ratio lower limit.

FIG. 4 illustrates a line graph 400 of the method 300 for selectivecurrent limiting of the chainsaw 100. The line graph 400 plots theaverage current 410, the PWM duty ratio 420, and the predeterminedcurrent threshold 430 over an operation time of the chainsaw 100. Asillustrated, the chainsaw 100 operates at a PWM duty ratio maximum limit440 at startup. The electronic processor 220 tracks the average current410 from startup. When the average current 410 exceeds the predeterminedcurrent threshold 430 at time T1, the electronic processor 220 reducesthe PWM duty ratio 420 in proportion to the difference between theaverage current 410 and the predetermined current threshold 430. Theelectronic processor 220 increases and decreases the PWM duty ratio 420based on the average current 410 exceeding the predetermined currentthreshold 430 as shown in the line graph 400. However, as shown at timeT2, the electronic processor 220 does not reduce the PWM duty ratiobelow the PWM duty ratio lower limit 450 even when the differencebetween the average current 410 and the predetermined current threshold430 exceeds the difference between the PWM duty ratio maximum limit 440and the PWM duty ratio lower limit 450. At time T3, when the averagecurrent 410 returns below the predetermined current threshold 430, theelectronic processor 220 increases the PWM duty ratio 420 back to thePWM duty ratio maximum limit.

In some embodiments, the current limiting method 300 is only performedwhen the current limiting is enabled by a user. As discussed above, theuser may enable or disable the current limiting feature using thecurrent limiting switch 210. As also noted above, the current limitingswitch 210 may be provided at various locations on the chainsaw 100, asa software button on a smart telephone application in wirelesscommunication with the chainsaw 100, or both. The electronic processor220 receives an indication whether the current limiting switch 210 isenabled or disabled from the current limiting switch 210 (on thechainsaw 100 or the smart telephone). The electronic processor 220determines based on the indication whether current limiting is enabled(or disabled). In response, the electronic processor 220 performscurrent limiting as described with respect to method 300 when thecurrent limiting switch 210 is enabled and does not perform the currentlimiting as described with respect to method 300 when the currentlimiting switch 210 is disabled. When the current limiting switch 210 isdisabled, the electronic processor 220 may not change the PWM duty ratioproportional to the average current and may simply turn off the chainsaw100 when the average or instantaneous current exceeds an absolutecurrent limit.

One advantage of the above methods is increased runtime of the powertool before a battery pack is depleted.

Thus, embodiments described herein provide, among other things,selectable current limiting for a power tool.

We claim:
 1. A power tool comprising: a housing; a motor within thehousing; a current sensor configured to measure a current flow to themotor; an inverter bridge selectively providing power from a powersource to the motor; and an electronic processor connected to thecurrent sensor and the inverter bridge, the electronic processor isconfigured to: determine, using the current sensor, an average current,determine whether the average current exceeds a predetermined currentthreshold, determine a deviation of the average current from thepredetermined current threshold, and reduce a pulse-width-modulated(“PWM”) duty ratio proportionally to the deviation of the averagecurrent from the predetermined current threshold, wherein the PWM dutyratio corresponds to a PWM signal provided to the inverter bridge. 2.The power tool of claim 1, wherein the average current is a slow-movingcurrent average determined by passing the current sensor readingsthrough a slow response filter.
 3. The power tool of claim 1, whereinthe average current is a fast-moving current average determined bypassing the current sensor readings through a fast response filter. 4.The power tool of claim 1, further comprising a trigger switch, whereinthe electronic processor is further configured to drive, using theinverter bridge, the motor at a maximum duty ratio when the triggerswitch is turned on.
 5. The power tool of claim 4, wherein the PWM dutyratio is not reduced below a PWM duty ratio lower limit even when thedifference between the average current and the predetermined currentthreshold exceeds the difference between the maximum duty ratio and thePWM duty ratio lower limit.
 6. The power tool of claim 4, furthercomprising a current limiting switch, wherein the electronic processoris configured to reduce the PWM duty ratio proportionally to thedeviation of the average current from the predetermined currentthreshold when the current limiting switch is turned on.
 7. The powertool of claim 6, wherein the electronic processor is configured to stopthe motor when the current limiting switch is turned off and the averagecurrent exceeds the predetermined current threshold.
 8. The power toolof claim 6, wherein the current limiting switch is provided on thehousing.
 9. The power tool of claim 6, wherein the current limitingswitch is provided on a graphical user interface of an external devicein wireless communication with the power tool.
 10. The power tool ofclaim 1, wherein the electronic processor is further configured to:determine the deviation of the average current from the predeterminedcurrent threshold as a percentage of maximum deviation; and reduce thePWM duty ratio by the percentage of maximum deviation.
 11. A method forselectable current limiting for a power tool, the method comprising:driving, using an inverter bridge of the power tool, a motor of thepower tool; determining, using a current sensor of the power tool, anaverage current flowing to the motor; determining, using an electronicprocessor of the power tool, whether the average current exceeds apredetermined current threshold; determining, using the electronicprocessor, a deviation of the average current from the predeterminedcurrent threshold; and reducing, using the electronic processor, a PWMduty ratio proportionally to the deviation of the average current fromthe predetermined current threshold, wherein the PWM duty ratiocorresponds to a PWM signal provided to the inverter bridge.
 12. Themethod of claim 11, wherein the average current is a slow-moving currentaverage determined by passing the current sensor readings through a slowresponse filter.
 13. The method of claim 11, wherein the average currentis a fast-moving current average determined by passing the currentsensor readings through a fast response filter.
 14. The method of claim11, further comprising driving, using the inverter bridge, the motor ata maximum duty ratio when a trigger switch of the power tool is turnedon.
 15. The method of claim 14, wherein the PWM duty ratio is notreduced below a PWM duty ratio lower limit even when the differencebetween the average current and the predetermined current thresholdexceeds the difference between the maximum duty ratio and the PWM dutyratio lower limit.
 16. The method of claim 14, wherein reducing the PWMduty ratio proportionally to the deviation of the average current fromthe predetermined current threshold is performed when a current limitingswitch is turned on.
 17. The method of claim 16, further comprisingstopping the motor when the current limiting switch is turned off andthe average current exceeds the predetermined current threshold.
 18. Themethod of claim 16, wherein the current limiting switch is provided on ahousing of the power tool.
 19. The method of claim 11, furthercomprising: determining the deviation of the average current from thepredetermined current threshold as a percentage of maximum deviation;and reducing the PWM duty ratio by the percentage of maximum deviation.20. A power tool comprising: a housing; a motor within the housing; acurrent sensor configured to measure a current flow to the motor; aninverter bridge selectively providing power from a power source to themotor; and an electronic processor connected to the current sensor andthe inverter bridge, the electronic processor is configured to:determine, using the current sensor, an average current, determinewhether the average current exceeds a predetermined current threshold,determine a deviation of the average current from the predeterminedcurrent threshold, and reduce a pulse-width-modulated (“PWM”) duty ratiobased on the deviation of the average current from the predeterminedcurrent threshold, wherein the PWM duty ratio corresponds to a PWMsignal provided to the inverter bridge.